1. Technical Field
The invention relates to upper internal nuclear reactor equipment for a reactor of the type comprising a pressure vessel closed by a removable cover and including at least one coolant input nozzle and at least one coolant output nozzle for the coolant situated in the vicinity of the cover, a core formed of fuel assemblies and in which the coolant flows upwardly, a plurality of control clusters actuated by drive shafts projecting through the cover of the reactor, said upper internal equipment including an assembly for collecting the coolant leaving the core toward the output nozzle, and a device for fractionating the coolant flow, including guides for the control clusters and drive shafts, inserted between the collection assembly or plenum and the core and including a lower plate formed with passages for the coolant leaving the core, an upper plate formed with passages for the coolant towards the collection assembly and a peripheral shroud connecting the plates together.
It is particularly, although not exclusively, suitable for use in pressurized water nuclear reactors, called PWRs. And it is of particular advantage in spectral shift reactors. In such reactors, the control clusters include clusters having rods which contain matter which exhibits parasitic neutron absorption, for adjusting the power of the reactor, and water displacement rods, preferably containing fertile material.
2. Prior Art
In the equipment of the above-defined type described in French No. 2,595,501, the flow separation or fractionating device includes spaced tubes connecting together the coolant passages in the upper and lower plates of the device and placed between the cluster guides.
That construction solves numerous problems. On the other hand, it is difficult to accommodate the mutual variations between the numerous spacer tubes. In the spectral shift reactors, where there is a large number of clusters, and consequently of cluster guides, the guiding structure above the core of the reactor is complex and construction is difficult.
In another prior art arrangement (European No. 0,222,373), the coolant flows axially within cluster guide shrouds. The high speed coolant sweeping the rods of each cluster, may vibrate the rods. The core upper plate must be quite thick to compensate for the lack of spacers.